(1) Field of the Invention
The present invention relates to a control method for controlling an overspeed safety system, to such an overspeed safety system, and to an aircraft including such an overspeed safety system.
The invention relates to the general technical field of systems that provide operating safety for aircraft engines, in particular operating safety for a twin-engine helicopter or rotorcraft.
The present invention relates more particularly to protecting a twin-engine power plant in the event of one of its two engines suffering overspeed.
(2) Description of Related Art
A twin-engine rotorcraft generally includes first and second engines acting together to drive a main rotor providing lift and possibly also propulsion via a power drive train.
The first and second engines are generally controlled by respective first and second control units. Such control units are known by the acronym FADEC for the term “full authority digital engine control”.
Each control unit may comprise a computer and control and monitoring peripherals constituting interfaces between the cockpit and the associated engine of the aircraft.
Among the kinds of engine concerned, particular use is made of free-turbine fuel-burning engines, in which a free turbine having at least one stage drives a shaft in rotation that engages with the power drive train.
In particular in the event of the power transmission train breaking or in the event of a freewheel in the power train slipping, the speed of rotation of the free turbine of an engine can increase considerably. The person skilled in the art then considers that such an engine is suffering overspeed, where such overspeed can lead to the engine running away and bursting.
Consequently, aircrafts are generally provided with safety systems to avoid overspeed in engines that might lead to events that are catastrophic for the aircraft and its occupants.
A mechanical system sometimes referred by the term “blade-shedding” may be applied to a free-turbine turbine engine.
Such a mechanical system consists in arranging shielding around the turbine and in facilitating the ejection of turbine blades as from a predetermined limiting speed of rotation.
Above the predetermined limit, a fuse element connecting each blade to a turbine hub breaks, for example. Each blade is then separated from the hub, with the blades nevertheless being contained inside the engine by the shielding.
That necessarily results in a reduction of the speed of rotation of the turbine, thus making it possible to stop the overspeed of the free turbine.
Engine overspeed is then stopped, but the engine is nevertheless partially destroyed.
An electronic system seeks to shut down the engines in order to avoid overspeed appearing.
The manufacturer then establishes a speed of rotation threshold for the free turbine in a turbine engine.
When the control unit of the engine detects overspeed, the control unit causes the engine to stop.
In that configuration, the engine is not damaged by the safety system.
Simultaneous overspeed of both turbine engines of an aircraft is also extremely rare.
Nevertheless, simultaneous overspeed may occur:
while the aircraft is performing a particularly severe maneuver causing both engines to reach the shut down threshold almost simultaneously, with both engines therefore being shut down;
in the event of failure of the electronics of the two control units; and/or
in the event of a loss of the two connections between the respective engines and the power train.
The aircraft may then be in a difficult situation insofar as all of its engines have been shut down.
In a variant, the shutting down of one of the engines is inhibited in the event of the other engine being shut down because of overspeed. After a first engine is shut down, if the second engine then suffers overspeed, the second engine is not allowed to actuate its overspeed protection.
Under such circumstances, the second engine is no longer protected against overspeed. Such a situation is improbable, but not impossible. Consequently, the second engine may subsequently be in an overspeed condition, and the second engine cannot be shut down automatically because its protection is inhibited. The second engine therefore runs the risk of running away and bursting.
In another variant, it is possible to reengage the electronic protection against overspeed for the second engine in order to avoid such a risk.
It is also possible to consider analyzing various parameters in the event of overspeed in order to determine the origin of the phenomenon and decide on whether or not to shut down the second engine on exceeding the shut-down threshold.
It is possible to detect that the increase in the speed of rotation of the turbine of an engine is temporary. Under such circumstances, it is not necessarily useful to shut down the engine.
Furthermore, it should be observed that the technological background includes document U.S. Pat. No. 4,638,781, which discloses a device for cutting off the fuel feed to an engine.
The technical background also includes the following documents: U.S. Pat. No. 4,500,966, U.S. Pat. No. 5,363,317, U.S. Pat. No. 5,948,023, and EP 1 739 436.
Documents U.S. Pat. No. 4,500,966 and U.S. Pat. No. 5,363,317 refer to operating limits.
Document U.S. Pat. No. 5,948,023 refers to overspeed testing.